U.S. patent application Ser. No. 449,655, filed Mar. 11, 1974, now U.S. Pat. No. 3,944,512, issued Mar. 16, 1976 (priority: Switzerland: 3620/73 of Mar. 13, 1973) relates to a process for the manufacture of heat-curable synthetic resins, based on reaction products of maleic anhydride with mixtures of polybutadiene, unsaturated hydrocarbon resins and unsaturated fatty acid glyceride esters, which can be diluted with water and are suitable for the electrophoretic coating process, characterised in that a mixture of:
A. 20-60% by weight of a polybutadiene having an average molecular weight of 750-2,000 and an iodine number between 300 and 450, PA1 B. 10-60% by weight of a polyisoprene resin having a viscosity between 30 and 800 cP (measured in 70% strength solution in toluene at 20.degree. C) and an iodine number between 160 and 400, and PA1 C. 5-40% by weight of a fatty acid glyceride ester, wherein the fatty acid radical contains 16-18 C atoms and the fatty acid glyceride ester has an iodine number between 140 and 220, is pre-polymerised by heating to 200.degree.-270.degree. C until the reaction mixture of components (a), (b) and (c), which has an initial viscosity of about 100-300 sec (measured according to DIN 4 sec 53,211), displays viscosities between 500 and 2,000 sec (measured according to DIN 4 sec 53,211), the resulting mixture is then reacted with PA1 D. 10-20% by weight of maleic anhydride at 180.degree. to 190.degree. C until no further free maleic anhydride is present and PA1 E. in the resulting adduct the anhydride groups present are opened by hydrolysis with water or by alcoholysis with the amount of monohydric alcohols with 1-4 C atoms required to form the half-ester. PA1 a. 20-60% by weight of a polybutadiene having an average molecular weight of 750-2,000 and an iodine number between 300 and 450, PA1 b. 10-60% by weight of a polyisoprene resin having a viscosity between 30 and 800 cP (measured in 70% strength solution in toluene at 20.degree. C) and an iodine number between 160 and 400, and PA1 c. 5-40% by weight of a fatty acid glyceride ester, wherein the fatty acid radical contains 16-18 C atoms and the fatty acid glyceride ester has an iodine number between 140 and 220, is pre-polymerised by heating to 200.degree.-270.degree. C until the reaction mixture of components (a), (b) and (c), which has an initial viscosity of about 100 to 300 sec (measured according to DIN 4 sec 53,211), displays viscosities between 500 and 2,000 sec (measured according to DIN 4 sec 53,211), the resulting mixture is then reacted with PA1 d. 10-20% by weight of maleic anhydride at 180.degree.-190.degree. C until no further free maleic anhydride is present and PA1 e. in the resulting adduct the anhydride groups present are opened by hydrolysis with water or by alcoholysis with the amount of monohydric alcohols with 1-4 C atoms required to form the half-ester, characterised in that in the reaction carried out to modify the process products, 5 to 25% by weight, relative to the total weight of the components (a), (b), (c) and (d), of adducts of .alpha.,.beta.-unsaturated aliphatic dicarboxylic acids to resin acids and/or adducts which are partially or completely esterified with polyhydric saturated aliphatic alcohols, are co-used as component (f). PA1 Component (a): 30 - 50% by weight, PA1 Component (b): 10 - 30% by weight, PA1 Component (c): 10 - 25% by weight, PA1 Component (d): 12 - 18% by weight and PA1 Component (f) 5 - 25% by weight. PA1 Component (a): 40 - 60% by weight, PA1 Component (b): 5 - 15% by weight, PA1 Component (c): 5 - 15% by weight, PA1 Component (d): 14 - 18% by weight and PA1 Component (f): 10 - 20% by weight.
The process of U.S. patent application Ser. No. 449,655 gives heat-curable synthetic resins which can be diluted with water, are suitable for the electrophoretic coating process and are distinguished by very good resistance to salt spray, good throwing powers, good film hardness and low sensitivity of the rinsed, but not yet stoved, films to drops of water. However, a continuing disadvantage of the binders described in U.S. patent application Ser. No. 449,655 is that at high deposition potentials, say above 300 volt, the binders tend to excess-coating, that is to say coating thicknesses of 30 .mu. and above are obtained on the metal sheets to be coated.
An object of the present invention is to provide a process for the manufacture of synthetic resins of this type which when used as binders no longer show this disadvantage and which, for these reasons, are distinguished by even better values of the throwing power and excellent voltage resistance. This means that the breakdown voltage, measured in volt, during electrical deposition is relatively high. The binders manufactured according to the invention should be capable of deposition over a wide voltage range without a danger of excess-coating, that is to say in coating thickness of 20-22 .mu..
In the process of the present invention, this is achieved by using, additionally to the components (a) to (d) of U.S. patent application Ser. No. 449,655, a component (f), which consists of an adduct of an .alpha.,.beta.-unsaturated aliphatic dicarboxylic acid to a resin acid, it being possible for these adducts to be partially or completely esterified with saturated polyhydric alcohols.